To earn this trophy, you need to upgrade one type of each weapon, though the weapon itself doesn't matter. Below a breakdown of how much money you may need.*: On some weapons, you may not need to pay for the level 1 upgrades. This is the default level and cost to buy the weapon. If you acquired it through other means rather than buying the final cost will be slightly lower.Baseball Bat - Melee ()

1. Level 1 - $250*
2. Level 2 - $2,500
3. Total - $2,750

KA-1 Kobra - Handgun ()

1. Level 1 - $100*
2. Level 2 - $100
3. Level 3 - $10,000
4. Level 4 - $20,000
5. Total - $30,200

D4th Blossom - SMG ()

1. Level 1 - $5,000*
2. Level 2 - $7,500
3. Level 3 - $15,000
4. Level 4 - $30,000
5. Total - $ 57,500

AS3 Ultimax - Shotgun ()

1. Level 1 - $10,000*
2. Level 2 - $10,000
3. Level 3 - $20,000
4. Level 4 - $40,000
5. Total - $80,000

K-8 Krukov - Rifle ()

1. Level 1 - $10,000*
2. Level 2 - $10,000
3. Level 3 - $20,000
4. Level 4 - $40,000
5. Total - $80,000

Annihilator - Explosive ()

1. Level 1 - $15,000*
2. Level 2 - $15,000
3. Level 3 - $30,000
4. Level 4 - $60,000
5. Total - $120,000

Reaper Drone - Special ()

1. Level 1 - N/A - This is a weapon you get for free via the storyline.
2. Level 2 - $30,000
3. Level 3 - $60,000
4. Level 4 - $120,000
5. Total - $210,000

The final cost of earning this trophy is around $580,450, so you'll be grinding for money for quite some time. There is a Friendly Fire store that you can buy in Loren Square, east of the medium Tank Mayhem and north of the Stikit Inn. This appears to be story related so you can buy it (early even) for only $100 ($500 hourly) and it gives a 10% discount, where as the rest cost $5,000 and only give a 5% discount. That will certainly make your upgrading a little less painful on the wallet.

If you still haven't fully upgraded your weapons, you can earn a 25% discount by finishing  kill-deckers.exe and choosing the weapons manufacturer door at the end. For some reason it doesn't work with the store mentioned above that you can buy for $100 and offers the 10% discount.

Note: This trophy is listed as buggy as it didn't pop when it should have. Now in theory it should pop when all weapons are upgraded, but it didn't. Fair enough, maybe the 4 types of grenades need upgrading too. However, upon upgrading the grenades but not all to max the trophy decided to pop. The only logical conclusion is that you need to upgrade the Flashbangs and only Flashbangs to level 4 where they become the Fart in a Jar. They are the only type of grenade that has a different characteristic added to it other than more damage and larger affect area.